1. Field of the Invention
This invention relates to a method of recognizing an irradiation field on a stimulable phosphor sheet bearing radiation image information recorded thereon by limitation of the irradiation field when the stimulable phosphor sheet is exposed to stimulating rays and light emitted by the stimulable phosphor sheet upon stimulation is photoelectrically detected to obtain image signals representing the radiation image information born by the stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, image processing is carried out on the electric image signal, and the radiation image of the object is reproduced as a visible image by use of the processed image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the system, in order to avoid influence of fluctuation in the recording conditions and to obtain a visible image suitable for viewing for diagnostic purposes, it is preferred that the characteristics of the stored image information determined depending, for instance, on the state of the radiation image information recorded on the stimulable phosphor sheet, the image recording portion of the object such as the head, chest or abdomen, and/or the image recording method such as plain image recording or contrasted image recording be ascertained prior to output of a visible image and the read-out gain be adjusted on the stored image information ascertained and the scale factor be determined according to the contrast of the stored image information so that an optimal resolution can be obtained.
Ascertaining of the characteristics of the image information stored on the stimulable phosphor sheet prior to output of the visible image may be carried out by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed method, a read-out operation for ascertaining the image information of a radiation image stored on the stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image (hereinafter referred to as the final read-out), thereby obtaining an outline of the characteristics of the stored image information, and the scale factor, the read-out gain and/or the signal processing conditions are determined on the basis of the information obtained by the preliminary read-out.
Various methods have been proposed for ascertaining the characteristics of the stored image information based on the preliminary read-out image signal obtained by the preliminary read-out or the final read-out image signal obtained by the final read-out. As one of such methods, it has heretofore been known to utilize a histogram of the image signals (image signal levels). With this method, the characteristics of the stored image information may be ascertained based on, for example, the maximum signal value, the minimum signal value, or a signal value at which the frequency is the maximum in the histogram. Therefore, it becomes possible to reproduce a visible image having an improved image quality, particularly a high diagnostic efficiency and accuracy, by adjusting the final read-out conditions such as the read-out gain and the scale factor and/or the image processing conditions such as the gradation processing conditions and the frequency response processing conditions based on the maximum signal value, the minimum signal value, a signal value at which the frequency is the maximum, or the like in the histogram.
On the other hand, in the course of radiation image recording, it is often desired that portions of the object not related to diagnosis or the like be prevented from exposure to a radiation. Further, when the object portions not related to diagnosis or the like are exposed to a radiation, the radiation is scattered by such portions to the portion related to diagnosis or the like, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in many cases, the irradiation field is limited to an area smaller than the overall recording region on the stimulable phosphor sheet when a radiation image is recorded.
However, when the characteristics of the image information stored on the stimulable phosphor sheet are ascertained based on the histogram of the image signals, the problem as described below arises. As shown in FIG. 2, when an irradiation field B is limited to an area smaller than an image recording region on a stimulable phosphor sheet 103 and the preliminary read-out or the final read-out is carried out over an area markedly larger than the irradiation field B, for example, over the overall image recording region on the stimulable phosphor sheet 103, the characteristics of the image information actually stored within the irradiation field B are ascertained incorrectly. Specifically, in the aforesaid case, since the image signals at regions outside of the irradiation field B are also included in the histogram, the histogram does not accurately represent the actual image information stored within the irradiation field B.
The applicants have proposed various methods of recognizing the irradiation field B as disclosed in, for example, U.S. Pat. No. 4,851,678. By recognizing the irradiation field according to such methods and carrying out the preliminary read-out only about the irradiation field, the problem described above can be overcome. However, with most of the proposed methods, recognition of the irradiation field is carried out assuming that the irradiation field is rectangular, and it is not always possible to accurately recognize the irradiation field in the case where it is of an irregular polygon or of a shape defined by a curve such as a circle, ellipse or the like.